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Macrophages and their counterparts in the central nervous system, the microglia, detect and subsequently clear microbial pathogens and injured tissue. These phagocytic cells alter and adapt their phenotype depending on their prime activity, i.e., whether they participate in acute defence against pathogenic organisms (‘M1’-phenotype) or in clearing damaged tissues and performing repair activities (‘M2’-phenotype). Stimulation of pattern recognition receptors by viruses (vaccines), bacterial membrane components (e.g., LPS), alcohol, or long-chain saturated fatty acids promotes M1-polarization. Vaccine or LPS administration to healthy human subjects can result in sickness symptoms and low mood. Alcohol abuse and abdominal obesity are recognized as risk factors for depression. In the M1-polarized form, microglia and macrophages generate reactive oxygen and nitrogen radicals to eradicate microbial pathogens. Inadvertently, also tetrahydrobiopterin (BH4) may become oxidized. This is an irreversible reaction that generates neopterin, a recognized biomarker for depression. BH4 is a critical cofactor for the synthesis of dopamine, noradrenaline, and serotonin, and its loss could explain some of the symptoms of depression. Based on these aspects, the suppression of M1-polarization would limit the inadvertent catabolism of BH4. In the current review, we evaluate the evidence that antidepressant treatments (monoamine reuptake inhibitors, PDE4 inhibitors, lithium, valproate, agomelatine, tianeptine, electroconvulsive shock, and vagus nerve stimulation) inhibit LPS-induced microglia/macrophage M1-polarization. Consequently, we propose that supplementation with BH4 could limit the reduction in central monoamine synthesis and might represent an effective treatment for depressed mood. [ABSTRACT FROM AUTHOR]

Highlights: [•] Antidepressant effects are mediated by 3-furan-2-yl-N-p-tolyl-acrylamide (PAM-2) after subchronic and chronic treatments. [•] This is the first time that an antidepressant activity is described for an α7 PAM. [•] The observed chronic and residual (i.e., after withdrawal) antidepressant activities are gender-dependent. [ABSTRACT FROM AUTHOR]

Abstract: The pharmacological properties of (±)-2-(N-tert-butylamino)-3′-iodo-4′-azidopropiophenone [(±)-SADU-3-72], a photoreactive analog of bupropion (BP), were characterized at different muscle nicotinic acetylcholine receptors (AChRs) by functional and structural approaches. Ca2+ influx results indicate that (±)-SADU-3-72 is 17- and 6-fold more potent than BP in inhibiting human (h) embryonic (hα1β1γδ) and adult (hα1β1εδ) muscle AChRs, respectively. (±)-SADU-3-72 binds with high affinity to the [3H]TCP site within the resting or desensitized Torpedo AChR ion channel, whereas BP has higher affinity for desensitized AChRs. Molecular docking results indicate that both SADU-3-72 enantiomers interact with the valine (position 13′) and serine (position 6′) rings. However, an additional domain, between the outer (position 20′) and valine rings, is observed in Torpedo AChR ion channels. Our results indicate that the azido group of (±)-SADU-3-72 may enhance its interaction with polar groups and the formation of hydrogen bonds at AChRs, thus supporting the observed higher potency and affinity of (±)-SADU-3-72 compared to BP. Collectively our results are consistent with a model where BP/SADU-3-72 and TCP bind to overlapping sites within the lumen of muscle AChR ion channels. Based on these results, we believe that (±)-SADU-3-72 is a promising photoprobe for mapping the BP binding site, especially within the resting AChR ion channel. [Copyright &y& Elsevier]

Abstract: The neuronal nicotinic acetylcholine receptor α7 (nAChR α7) may be involved in cognitive deficits in Schizophrenia and Alzheimer''s disease. A fast pharmacological characterization of homomeric α7 receptors is mostly hampered by their low functional expression levels in heterologous expression systems. In the present study expression of homomeric nAChR α7 was achieved in GH3 rat pituitary cells. α7 Subunits were heterologously expressed as components of [125I]-labeled α-bungarotoxin binding nAChRs (B max: 1.2pmol/mg protein). Function of the expressed α7 ion channels was assessed by patch-clamp recording and calcium imaging. While acetylcholine-induced currents desensitized within much less than 1s, calcium-sensitive fluorescence transients peaked after 5–10s and returned to background levels within 30s only. The fluorescence signal was blocked by isradipine and removal of extracellular sodium indicated that in these cells opening of rapidly desensitizing α7 nAChR triggers calcium influx via voltage-gated, DHP-sensitive calcium channels. In this cellular system, agonists revealed the following rank order of potency: epibatidine>anatoxin A>AAR17779>ABT-594>DMPP>nicotine>GTS-21>cytisine>ABT-418>acetylcholine>choline>ABT-089. All of the signals were inhibited by the α7 antagonists α-bungarotoxin (pIC50: 7.4) and methyllycaconitine (pIC50: 7.8). Further, marketed antidepressants showed antagonistic activity with the following rank order of potency: fluoxetine>imipramine>paroxetine>sertraline. These data illustrate that coupling to voltage-gated calcium channels allows a rapid and reliable functional examination of nAChR α7. [Copyright &y& Elsevier]

The current study compares the antidepressant-like effect elicited by nicotine between wild-type (β4+/+) and knockout (β4−/−) mice, and subsequently, the effect of 3-furan-2-yl- N - p -tolyl-acrylamide (PAM-2), a positive allosteric modulator of α7 nicotinic receptors, on the previously determined activity of nicotine. Mice from each sex were injected daily with nicotine base (0.2 mg/kg; s.c.) or co-administered with PAM-2 (1.0 mg/kg; i.p.) for 3 weeks. Forced swim tests were performed to determine the acute (day 1), subchronic (day 7), and chronic (days 14 and 21) effects of the drugs, as well as their residual effects after treatment cessation (days 28 and 35). Our results indicate that nicotine mediates antidepressant-like activity after acute, subchronic, and chronic treatments in β4+/+, but not β4−/−, mice, and that these effects are not mediated by unspecific locomotor stimulation. Nicotine co-administered with PAM-2 produces antidepressant-like activity in both β4+/+ and β4−/− mice, except after the acute treatment of β4−/− mice, and decreases locomotor activity. This suggests that although the β4 subunit regulates the antidepressant-like activity of nicotine it does not affect the activity elicited by PAM-2 when is co-administered with nicotine. The residual antidepressant-like activity of PAM-2 + nicotine was observed only in female mice, suggesting gender-specific differences. Our findings clearly indicate that β4-containing nAChRs play an important role in the antidepressant-like activity elicited by nicotine but they are not essential for the modulatory activity of PAM-2. In fact, PAM-2 inhibits α4β4 and α3β4 AChRs at higher concentration ranges compared to that for the PAM activity previously found at the α7 AChR. [ABSTRACT FROM AUTHOR]

The pharmacological activity of bupropion was compared between α7 nicotinic acetylcholine receptors expressed in heterologous cells and hippocampal and dorsal raphe nucleus neurons. The inhibitory activity of bupropion was studied on GH3-α7 cells by Ca 2+ influx, as well as on neurons from the dorsal raphe nucleus and interneurons from the stratum radiatum of the hippocampal CA1 region by using a whole-cell voltage-clamp technique. In addition, the interaction of bupropion with the α7 nicotinic acetylcholine receptor was determined by [ 3 H]imipramine competition binding assays and molecular docking. The fast component of acetylcholine- and choline-induced currents from both brain regions was inhibited by methyllycaconitine, indicating the participation of α7-containing nicotinic acetylcholine receptors. Choline-induced currents in hippocampal interneurons were partially inhibited by 10 µM bupropion, a concentration that could be reached in the brain during clinical administration. Additionally, both agonist-induced currents were reversibly inhibited by bupropion at concentrations that coincide with its inhibitory potency (IC 50 =54 µM) and binding affinity ( K i =63 µM) for α7 nicotinic acetylcholine receptors from heterologous cells. The [ 3 H]imipramine competition binding and molecular docking results support a luminal location for the bupropion binding site(s). This study may help to understand the mechanisms of actions of bupropion at neuronal and molecular levels related with its therapeutic actions on depression and for smoking cessation. [ABSTRACT FROM AUTHOR]

Highlight: [•] (-)-Reboxetine inhibits muscle AChRs by a non-competitive mechanism. [•] (-)-Reboxetine blocks AChR function by binding to the TCP luminal site. [•] (-)-Reboxetine induces AChR desensitization probably by interacting with non-luminal sites, a mechanism that is shared by tricyclic antidepressants. [Copyright &y& Elsevier]

Abstract: The interaction of tricyclic antidepressants (TCAs) with the human (h)α3β4 nicotinic acetylcholine receptor (AChR) in different conformational states was compared with that for mecamylamine by using functional and structural approaches including, Ca2+ influx, radioligand binding, and molecular docking. The results established that: (a) [3H]imipramine binds to a single site with relatively high affinity (K d =0.41±0.04μM), (b) imipramine inhibits [3H]imipramine binding to the resting/κ-bungarotoxin-bound AChR (K i =0.68±0.08μM) with practically the same affinity as to the desensitized/epibatidine-bound AChR (K i =0.83±0.08μM), suggesting that TCAs do not discriminate between these conformational states, and (c) although TCAs (IC50 ∼1.8–2.7μM) and mecamylamine (IC50 =3.3±0.4μM) inhibit (±)-epibatidine-induced Ca2+ influx with potencies in the same concentration range, TCAs (K i ∼1–3.6μM), but not mecamylamine (apparent IC50 ∼0.2mM), inhibit [3H]imipramine binding to hα3β4 AChRs in different conformational states. This is explained by our docking results where imipramine, in the neutral and protonated states, interacts with the leucine (position 9′) and valine/phenylalanine (position 13′) rings, whereas protonated mecamylamine (>99% at physiological pH) interacts with the outer ring (position 20′). Our data indicate that TCAs bind to overlapping sites located between the serine and valine/phenylalanine rings in the hα3β4 AChR ion channel, whereas protonated mecamylamine can be attracted to the channel mouth before blocking ion flux by interacting with a luminal site in its neutral state. [Copyright &y& Elsevier]

The activity of tricyclic antidepressants (TCAs) at α7 and α9α10 nicotinic acetylcholine receptors (AChRs) as well as at hippocampal α7-containing (i.e., α7*) AChRs is determined by using Ca 2+ influx and electrophysiological recordings. To determine the inhibitory mechanisms, additional functional tests and molecular docking experiments are performed. The results established that TCAs (a) inhibit Ca 2+ influx in GH3-α7 cells with the following potency (IC 50 in μM) rank: amitriptyline (2.7 ± 0.3) > doxepin (5.9 ± 1.1) ∼ imipramine (6.6 ± 1.0). Interestingly, imipramine inhibits hippocampal α7* AChRs (42.2 ± 8.5 μM) in a noncompetitive and voltage-dependent manner, whereas it inhibits α9α10 AChRs (0.53 ± 0.05 μM) in a competitive and voltage-independent manner, and (b) inhibit [ 3 H]imipramine binding to resting α7 AChRs with the following affinity rank (IC 50 in μM): imipramine (1.6 ± 0.2) > amitriptyline (2.4 ± 0.3) > doxepin (4.9 ± 0.6), whereas imipramine’s affinity was no significantly different to that for the desensitized state. The molecular docking and functional results support the notion that imipramine noncompetitively inhibits α7 AChRs by interacting with two overlapping luminal sites, whereas it competitively inhibits α9α10 AChRs by interacting with the orthosteric sites. Collectively our data indicate that TCAs inhibit α7, α9α10, and hippocampal α7* AChRs at clinically relevant concentrations and by different mechanisms of action. [ABSTRACT FROM AUTHOR]

Abstract: The interaction of tricyclic antidepressants with the human (h) α4β2 nicotinic acetylcholine receptor in different conformational states was compared with that for the noncompetitive antagonist mecamylamine by using functional and structural approaches. The results established that: (a) [3H]imipramine binds to hα4β2 receptors with relatively high affinity (K d =0.83±0.08μM), but imipramine does not differentiate between the desensitized and resting states, (b) although tricyclic antidepressants inhibit (±)-epibatidine-induced Ca2+ influx in HEK293-hα4β2 cells with potencies that are in the same concentration range as that for (±)-mecamylamine, tricyclic antidepressants inhibit [3H]imipramine binding to hα4β2 receptors with affinities >100-fold higher than that for (±)-mecamylamine. This can be explained by our docking results where imipramine interacts with the leucine (position 9′) and valine (position 13′) rings by van der Waals contacts, whereas mecamylamine interacts electrostatically with the outer ring (position 20′), (c) van der Waals interactions are in agreement with the thermodynamic results, indicating that imipramine interacts with the desensitized and resting receptors by a combination of enthalpic and entropic components. However, the entropic component is more important in the desensitized state, suggesting local conformational changes. In conclusion, our data indicate that tricyclic antidepressants and mecamylamine efficiently inhibit the ion channel by interacting at different luminal sites. The high proportion of protonated mecamylamine calculated at physiological pH suggests that this drug can be attracted to the channel mouth before binding deeper within the receptor ion channel finally blocking ion flux. [Copyright &y& Elsevier]